1. Field of Invention
The invention relates to an electrochemical process performed with a constant, regulated voltage of the test electrode for the selective determination of in particular peracetic acid mixed with other oxidants, such as e.g. H.sub.2 O.sub.2, as well as other substances, a corresponding potentiostatic apparatus for performing the process and the use of the process for the continuous measurement of process flows containing peracetic acid.
2. Description of the Prior Art
The strict requirements made regarding the quality control of foods and beverages (cf. minimum keeping quality) inter alia make necessary the most careful procedures when cleaning and disinfecting the corresponding production and processing plants, e.g. for milk, beer and soft drinks. In order to ensure the necessary sterility the containers, pipes, pumps, filters and filling devices must be regularly treated with cleaning agents (e.g. acids and lyes) and disinfecting solutions. The said plants must then be rinsed free with clean water in order to avoid inadmissible actions on the products by harmful impurities. For this purpose use is frequently made of CIP processes in various technical forms. The cleaning and disinfecting solutions can be discarded after passing through the plants (usually in the case of smaller systems) or can be returned to holding tanks for rational reuse. At this point there is a supplementing of the spent active substances.
Prior to entering the plants the disinfectants must have the concentrations necessary for an adequate activity. It can also be useful to know the active substance losses in the individual plant parts and/or in the drains. For rational water consumption purposes it is useful to determine the time of adequate free rinsing of the plant. It is necessary to prove that the actual products are not contaminated. It is important for the corresponding analysis which disinfectants are used for solving the complex measurement problem.
Peracetic acid (CH.sub.3 COOOH), mixed with hydrogen peroxide (H.sub.2 O.sub.2) and acetic acid (CH.sub.3 COOH), is being increasingly used as a strong, oxidatively acting disinfectant. It therefore replaces chlorine-containing substances, quaternary ammonium compounds and expensive ozone. The broad action spectrum against microorganisms (bacteria, fungi, viruses), even at low temperatures, the usually problem-free reduction product acetic acid (CH.sub.3 COOOH.fwdarw.CH.sub.3 COOH+1/2 O.sub.2), as well as the complete degradability thereof in biological sewage treatment plants are excellent and convincing advantageous characteristics, in addition to the relatively easy production process.
Concentrates with different active substance concentrations are marketed on the basis of peracetic acids. Corresponding to the production from high percentage hydrogen peroxide and concentrated acetic acid, equilibrium concentrations are obtained with a peracetic percentage between 3 and 40%. The addition of stabilizers (often phosphonic acids) slows down the self-decomposition of these substances.
To part of said commercial products are also added mineral acids (sulphuric/nitric acid). The disinfecting solutions used are prepared from these concentrates by diluting in a ratio of 1:100 to 1:1000, giving peracetic acid concentrations between 100 and 2000 ppm (mg/l).
For the analytical determination of peracetic acid concentrations in these ranges, use is almost exclusively made of a titration process. In the latter, firstly by titration with potassium permanganate solution the H.sub.2 O.sub.2 content is determined and then eliminated. This is followed by peracetic acid titration with sodium thiosulphate solutions against iodine strength as the indicator. Complicated photometric determination methods or the evaluation of luminescence effects are too complicated and costly for operational measuring technology in connection with disinfecting measures.
The use of test strips with colour indicators is simple and fast, but often requires dilution and the results are often not very accurate.
For the continuous measurement of the peracetic acid, which is extremely important for operationally reliable and rational use, no suitable process has been available up to now. Solely for the detection of the risk of the contamination of beverages, use is made operationally of conductivity measuring processes, but these require the aforementioned addition of mineral acids to the disinfectants.
German Patent DE 4,223,228 A1 describes a cyclovoltametrically operating process for the determination of per acids, as well as H.sub.2 O.sub.2, in which the potential difference of two electrodes in the central third of the double layer range is chosen. Through a voltage-dynamic procedure a cathodic maximum of the current signal which occurs is determined and which is essentially associated with the per acid content of the electrolyte, i.e. the solution. The optimum potential difference must be cyclovoltametrically, iteratively determined.
The current-voltage characteristic of this potentiodynamic procedure performed with high change speeds for the voltage in the double layer range mainly undergo determination by capacitive currents and processes of adsorption and desorption, which are accompanied by charge conversion.
Thus, in summarizing, it can be established that hitherto no suitable measuring process is known, which is able to solve the aforementioned measuring problems, particularly through the indication of a constantly up-dated measured value.
Therefore the problem of the invention is to be able to measure peracetic acid with high excesses (up to 10 times and more) of other oxidants such as H.sub.2 O.sub.2 in a selective and continuous manner, particularly in a peracetic acid concentration range of 10 to 2000 ppm.